It would be "nonsensical and offensive" to suggest that if MI5 knew about a terrorist attack on London it would not take steps to prevent it, Witness G, chief of staff to MI5 head Jonathan Evans, told the inquests into the July 7 bombings. MI5 had "no inkling" of what was to befall London in the summer of 2005, Witness G agreed in answer to leading questions at the start of his evidence.

For the families of the 52 victims and the 700 injured, this was unlikely to have been the point. The question was whether MI5 failed to pursue potential leads that might have led it to the four suicide bombers.

The coroner's report contains some tantalising passages, references to incidents preceded by the word "fortunately", and "unfortunately", and to the occasional "mistake". These passages may have left the families frustrated, a feeling that questions remain unanswered as coroner Lady Hallett concluded, without equivocation, that MI5, faced with threats to the UK that were "immense" in both scale and number, could not be blamed.

MI5 had frequently referred to the advantages of hindsight, and its finite resources – by implication, far too small at the time. To the service's relief, Lady Hallett gave it the benefit of any doubts that may be lingering. On choosing what the priority targets should be and what she called the "structure of decision-making", she said: "I feel these are very much areas best left to the experts." Responding to evidence of problems MI5 and the police had with computer-based data systems, she said: "This is an area where I again feel I must leave it to the experts."

Such language may not have made the families think the coroner was being as robust as she should have been with the Security Service. MI5, however, may be stung by Hallett's decision to make two recommendations about the agency, dismissing its insistence in closing submissions to the coroner that they would imply its "practices and methods of operation were defective today". It added: "On the evidence, there is simply no basis for concluding that there are any current circumstances, in 2011, relating to the work or practices of the Security Service, which create a risk to life."

There is another such circumstance, the whole question of accountability. MI5 wanted to submit its evidence to the inquests in private but failed to convince the coroner. In a dig at MI5, Hallett said: "To my mind, the concerns that I would not be able to conduct a thorough and fair investigation into the security issues in wholly open evidential proceedings have proved unfounded."

She pointed to "misleading" information that MI5 "inadvertently" gave to the Intelligence and Security Committee (ISC). The coroner said: "It is unfortunate to say the least that a body established by parliament to review the work of the Security Service, in closed hearings, reported inaccurately in these regards and that these points were not corrected", the coroner said.

Sir Malcolm Rifkind, former senior Conservative minister and chair of the ISC, said it was conducting a "root and branch" inquiry into how to give it more powers. It was "very unsatisfactory" that the committee was able only to request information rather than require the security and intelligence agencies to provide it, he added. Andrew Tyrie, a senior Tory MP and chairman of the all-party committee on extraordinary rendition – an issue the ISC also failed to get to the bottom of – also called for more effective scrutiny of the agencies.